This invention relates to a method and apparatus for distributing a centrally generated signal to several signal receivers. For example such a device is used in electromagnetic multi-element antennas in which a centrally generated signal is distributed to the transmitting and receiving modules of the antenna.
When signals are transmitted, the phase at the end of the transmission path differs from that of the input signal as a function of the signal frequency and the signal transit time. During operation, the signal is subjected to external disturbing influences (such as a change of the thermal environment), which, when the signal frequencies are high and/or the signal paths are long, may result in unacceptably high phase errors. (Signals may, for example, be electric, electromagnetic, acoustic or other type of radiation, and are carried in suitable media--herein called lines.) The signal speed in the conductor may be influenced, for example, by the ambient temperature, ambient pressure, mechanical deformations, tensile, pressure, shearing and bending stresses, electric and magnetic fields, acoustic waves or radiation of different origins.
Currently, basically two different approaches are used to suppress externally caused perturbation of the propagation duration along the lines in the different channels of a network. On the one hand, the network is shielded from external disturbances, or their influence is reduced, by means of special casings, which often results in considerable additional (generally mechanical) expenditures. Another approach consists of individually delaying the signal for each line in such a manner that, after the transmission, the signal passes through the nominal overall delay. In the case of electromagnetic multi-element antennas, such as satellite-supported large-surface synthetic aperture radar ("SAR") antennas, this approach is often implemented by means of phase adjusters. For this purpose, it is necessary to determine the actual signal transit time in the line, which must be measured in calibration cycles for each line. For example, in X-band antennas, the transit time must be stable to better than 1 ps, which corresponds to a propagation path in the vacuum of 0.3 mm.
It is an object of the present invention to provide a signal distribution network, in which external interference can be compensated with as little expenditure as possible, and in particular without calibration cycles or shielding.
This object is achieved by the network for distribution of a centrally generated signal to several signal receivers according to the invention, in which each path segment exhibits a sensitivity to external influences which can be determined at the time of the design. This location-x-dependent (or location-s-dependent) influence or disturbance (called T(x)) may be, for example, the temperature distribution along the line. (The coordinate x represents the location along the path followed by the lines, while the coordinate s represents the location along the line itself. That is, as explained below, the exact routing (and hence length) of the line itself within the path x may be varied, so that s=f(x).) The invention, recognizes and takes into account the fact that while the influence of a T(x)-fluctuation cannot be suppressed or cancelled out, effective suppression can be achieved in the action of lines relative to one another.
According to the invention, the arrangement comprises the following characteristics:
The lines follow a common path x, from which branches exit for the individual signal receivers. As a result, the lines are subjected to the same external influences. PA1 Within the common path x, the individual lines s extend beyond the i-th signal receiver to which they are coupled, to the end of the common path, and then are turned back on themselves (looped) in such a manner that the path segment from the signal source to the i-th signal receiver (signal receivers along the common path being numbered sequentially by consecutive indices i) is traversed by an odd number of strands of the line s, while the path segment from the i-th signal receiver to the end of the common path is traversed by an even number of strands. PA1 The individual lines are divided into two segments which have different relative sensitivities; and PA1 For each path segment .increment.x along the common path, the number of strands of a line multiplied by the relative sensitivity E of the line is a constant for all lines.
For the purpose of the foregoing, relative sensitivity (sometimes referred to as relative propagation sensitivity) E can be derived as follows:
The total transit time of a signal through a line s along path x, given a propagation speed c(T(s)), is as follows: ##EQU1## which may also be expressed as a function of the path x as: ##EQU2##
and C.sub.o (T(x))=propagation speed of the signal in a standard line--as a function of the external influence or disturbance T(x) at location x
and p=ratio between the propagation speed of the signal in a line which has been altered (for example, by doping) relative to that of a standard line selected for C.sub.o (T(x)), so that ##EQU3##
For most purposes it is satisfactory to calculate p as the ratio of the derivatives of c.sub.o (T) and c(T) with respect to the disturbance T: ##EQU4##
The product s'*p in Equation 2 above is referred to as relative sensitivity E. The incremental path element ds is measured along the line itself, while the coordinate x is related to distance along the common path (that is, the path followed by the grouping into which the lines are combined).
The relative sensitivity E can be adjusted in two ways:
1. As noted previously, the relationship of the line length increment ds to the path length increment dx can be adjusted when the line path has sufficient freedom of design. That is, for example, if x follows a relatively "straight" path for the grouping of lines, values for ds/dx which are greater than or equal to 1 can be adjusted by looping and winding the locus of the line s within path x. A uniform ds/dx along the path x can be achieved, for example, by laying the line in a constant diameter, constant slope helix within the path x. The ratio ds/dx can be adjusted by altering the slope of the helix. It may also be advantageous to reverse the rotational direction of the helix within the path x when external influences are to be suppressed (for example, in the case of electromagnetic irradiation).
2. The relative sensitivity E may also be adjusted by selection of the line material or by suitable doping (variation of p), so that the number of wave cycles of the signal per unit of length is changed.
Applications of the invention relate particularly to groupings of lines which terminate at different locations, and which relative to one another upon varying T(x) change their phase propagation speed as little as possible across each cross section. In the case of SAR-antennas which are subjected to extreme thermal fluctuations, a network therefore can be achieved in which phase fluctuations or transit time fluctuations of the individual channels relative to one another are suppressed, without need of special shielding, and without constant reconfiguration of the network. In addition, the arrangement according to the invention can be used for the distribution of signals having high carrier frequencies, such as optical signals, with drastically reduced sensitivity to outside disturbances.
It should be noted that it is unimportant in this case whether the lines are centrally guided together again. Also, the network may be operated in either direction, so that in addition to signal distribution, the arrangement according to the invention may also be used for collection of signals (in other words, signal reception in the case of a multi-element antenna). In the latter case (collection of signals), the central signal source is replaced by a central receiver, and the signal receivers are replaced by devices for signal input.
With respect to the grouping of lines, it should also be noted that the disturbance T(x) varies little in the transverse direction at any location along the grouping. The quality of this uniformity, of course, depends on the diameter, on the matrix into which the lines are embedded as shown in FIG. 7 and on the physical variation of the disturbance T. For example, heat-conducting materials are suitable for the embedding and a heat-insulating layer is very suitable as a covering in order to reduce thermal gradients, such as shown, for example, in FIG. 7 wherein a plurality of lines are embedded in a matrix, which is surrounded by an insulating layer. See also, FIG. 5 which shows a plurality of lines surrounded by an insulation layer.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.